This invention relates to a method and apparatus for moving the read/write head of a magnetic disc drive between a takeoff and a landing zone on the magnetic disc when the disc is starting and stopping. Wear resulting from contact of a head on a disc is thus spread over two regions, thereby slowing the rate at which wear accumulates in either region.
Disc drive heads fly on a thin film of air and do not contact the rotating disc they are flying over. During starting and stopping, and while the disc is stopped, the heads do come in contact with the disc. Contact of the heads with the disc causes wear. If the heads make contact with a portion of the disc that contains data, the eventual result is destruction of that data.
To prevent the possibility of destruction of data by the head, a dedicated track can be reserved on the disc for contact by the head. The location contains no data, and therefore does not present the risk of loss of data. To save valuable space on the disc, the dedicated track is made as narrow as possible. This has the effect of concentrating the disc wear caused by the heads in a very narrow band defined by the width of the rails on the recording head.
The use of a single narrow dedicated track prevents destruction of data due to contact of the disc by the head, and the narrowness of the track minimizes the area of the disc that cannot carry data. However, restricting contact of the heads with the disc to a single area concentrates all the wear of the disc by the heads to that area. Only one area of the disc is subject to wear, but that area is subjected to a much greater rate of wear than if the wear were distributed over a greater portion of the disc. The concentration of wear that occurs when the heads are restricted to contact with a single small area of the disc limits the number of cycles the disc drive can sustain before the disc media fails.
Dedication of a landing zone allows that a certain degree of wear can be tolerated. This is true because no data is present on the dedicated landing zone, and therefore the wear to which the dedicated landing zone is subject will not destroy any data. Nevertheless, wear of the dedicated landing zone will eventually render the disc unusable. As the landing zone wears, it becomes more and more irregular. The recording head must contact the landing zone; therefore, as the landing zone becomes more worn, the irregularity of the landing zone subjects the recording head to a greater rate of wear, an increased contamination risk, and other negative effects. It therefore becomes desirable to reduce the wear to which the landing zone is subjected.
One approach to distributing wear more evenly over the disc is to use a loose latch where the actuator latching mechanism has surfaces that permit the actuator a degree of freedom in starting or stopping. This permits a degree of randomness in location of contact of the disc by the head, thus distributing the wear more evenly and lengthening the life of the disc. This technique does not achieve even distribution, however, because of two factors.
The first factor is that the flexible actuator does not provide an equal probability for starting and stopping at all possible locations. Rather, the starting and stopping locations are distributed according to a Gaussian curve. The wear is thus concentrated near the center of the curve. The second factor is that during the time the disc is rotating, the actuator is subjected to forces that tend to bias the actuator against one side of the start/stop band. This further concentrates wear on that side.
U.S. Pat. No. 4,897,743, issued Jan. 30, 1990 to Kohno, discloses a control system for moving the heads at startup of the disc drive. When the disc drive unit is turned on, the disc drive motor and the actuator control is directed to move the head. Because the inertia of the disc is relatively large, there is a delay before the rotation of the disc begins. By moving the heads prior to disc rotation, Kohno may damage the head, disc, or flexible load beam. The actuator control responds more quickly and moves the head before the rotation of the disc begins.
The Kohno patent moves the head in order to break the adhesion of the head to the disc to prevent damage to the head or the disc once rotation begins. It is not important to the operation of the Kohno patent to place the head in any particular position.